223] J. T. Sing ew aid 25 



thought of. The time has arrived for a definite recognition 

 of these discordant data and a remolding of our conceptions 

 in harmony with them. 



The term magmatic segregation was borrowed by the eco- 

 nomic geologist from the petrographer and used in the petro- 

 graphic sense. Its application to the explanation of the 

 genesis of certain ore deposits seemed very plausible. It is 

 a matter of common petrographic knowledge that no large 

 body of igneous rock is of uniform composition and that 

 frequently the composition of a portion of the mass departs 

 widely from the average. Consequently there were forces at 

 work prior to or during the consolidation of the molten 

 magma which caused local segregations of certain of its con- 

 stituents. The nature of these forces has long been a matter 

 of discussion and speculation but unanimity of opinion has 

 not been attained and there is no thoroughly satisfactory 

 explanation of the process which is known as a magmatic 

 segregation or differentiation. The usual manifestation of 

 the phenomenon is in the local accumulation of the more basic 

 constituents of the magma. 



The basis for the application of this process as an explana- 

 tion of ore genesis rests on certain other observations in the 

 field of petrography. There is present in almost all igneous 

 rocks a group of opaque minerals, occurring as accessory con- 

 stituents and with euhedral forms, the most common repre- 

 sentatives of which are the sulphides of iron, frequently cu- 

 priferous, and the oxides of iron, frequently chromiferous or 

 titaniferous. On account of their commonly euhedral forms, 

 these minerals are regarded by petrographers as the earliest 

 constituents of the magma to crystallize. Furthermore, these 

 minerals are concentrated together with the basic silicates in 

 the process of rock differentiation. 



There are many ore deposits world-wide in their distribu- 

 tion possessing certain common characteristics, among which 

 may be mentioned: (1) the ore minerals consist of one or 

 more of the accessory opaque minerals common to igneous 

 rocks; (2) the enclosing rock is always an igneous rock and 



